Recently, there are proposed some semiconductor-mounting methods compatible with the demand for increase in performance and density of semiconductor elements and in mounting density. Typical of them are a COL (chip on lead) method of mounting a semiconductor chip on a lead frame having no die pad and a LOC (lead on chip) method of mounting a lead on a semiconductor chip.
Paste adhesive agents of a epoxy-based or acrylic resin and heat-resistant materials carrying a coated adhesive agent are commonly used as adhesive materials for bonding a semiconductor element on lead frame in these methods. However, these adhesives demand heating at higher temperature for an extended period for hardening, and use of them is not favorable from the point of productivity. In addition, these adhesives, which are relatively more hygroscopic, cause problems such as deterioration in the reliability of semiconductor package.
To solve the problems above, proposed is a method of improving productivity by using an adhesion film for semiconductor of polyimide resin that does not demands heating and hardening for an extended period of time (see, for example, Japanese Patent Application Publication No. 5-105850). However, the method demands adhesion at extremely high temperature, which is likely to cause thermal damage in the semiconductor element. On the other hand, it is possible to decrease the temperature of adhesion by lowering the glass transition temperature of the polyimide resin, but, in such a case, the decrease in glass transition temperature leads to deterioration in heat resistance of the polyimide resin at the same time and easier generation of voids during adhesion, and consequently to deterioration in adhesive strength. Further, it may also affect the wire-bonding efficiency and the reflow resistance in the production steps for a semiconductor package, consequently leading to deterioration in reliability of the semiconductor package.
To cope with the recent demand for further increase in density and reduction in the area and thickness of semiconductor packages, also under development are packages in the structure in which only one (semiconductor element-sided) face of the package is sealed and the lead frame exposed on the other face is used as a lead terminal. The package in such a structure is advantageous for reducing the area and thickness, because the lead frame is not sticking out of the sealing resin, but often causes problems such as migration of sealing resin to the rear face of the lead frame during sealing. In contrast, the inventors had found that it was possible to prevent the problems above by protecting the lead frame exposed on the rear face with an adhesion film and removing the layer after sealing, but, in recent popularization of various surface treatments such as coating of an organic film on the lead frame surface, there is still a problem of deterioration in the adhesive strength at the interface between the adhesion film and the lead frame.